1. Field of the Invention
The present invention relates to organic EL (electroluminescence) elements and to a process for their fabrication.
2. Related Background Art
The announcement by Kodak Corp. of a laminated organic EL element having an organic layer formed by vacuum vapor deposition using a low molecular compound eventually led to energetic development of organic EL displays, many of which are currently being implemented. At the same time, organic EL elements employing luminescent polymer compounds in the organic layers, which exhibit excellent thermal stability, are also being actively developed. Using a polymer compound in the organic layer not only produces excellent thermal stability but also allows the organic layer to be formed by a coating method using a coating solution, thus affording the advantage of simplifying the production process. In addition, an organic layer formed using vacuum vapor deposition includes sections with no layer formation, often leading to defects, whereas an organic layer formed by coating is highly resistant to defects and is highly reliable.
Nevertheless, when laminating an organic EL element using a polymer compound, it is necessary to select, as the solvent of the coating solution used for formation of the organic layer, a solvent which does not notably affect the properties of the polymer compounds in the other layers. Such solvent selection is very difficult, and therefore as the organic EL element using a polymer compound, an element having a single organic layer structure is usually employed, except when using PEDOT/PSS, which is a water-dispersible material which can be used for the hole injection layer.
An organic EL element having a single organic layer structure must have the single organic layer exhibiting the function of carrier transport, carrier recombination and luminescence. However, organic EL elements employing conventional polymer compounds do not effectively exhibit all of these functions due to carrier balance shift and the like, and they have therefore often been inadequate from the standpoint of element characteristics such as luminescent efficiency, heat resistance and usable life. A demand has existed, therefore, for an organic EL element employing a polymer compound with excellent element characteristics, from the viewpoint of production process reliability (yield) and production cost.
Several laminated organic EL elements using polymer compounds in the organic layers have been proposed to meet this demand. For example, Japanese Patent Application Laid-Open No. 2000-150165 proposes an organic EL element wherein a solution comprising a mixture of an alcohol-soluble buffer polymer and an electron transport material is coated to form an electron transport layer on the luminescent layer.
Also, Japanese Patent No. 2793383 proposes an organic EL element having an insulating metal oxide with an energy gap of 4.0 eV or greater inserted between the luminescent layer and the cathode, for the purpose of improving adhesion of the metal used in the cathode to permit uniform luminescence, and for increased recombination between the electrons and positive holes.